1. Technical Field
The present disclosure relates to the production of wellbore fluids, such as a process for making a polymeric additive for a wellbore fluid.
2. Background
The present invention relates to a low fluid loss aqueous-based wellbore fluid, for example, a drilling fluid, completion fluid, workover fluid or packer fluid. Conventionally, the drilling of a well into the earth by rotary drilling techniques involves the circulation of a drilling fluid from the surface of the earth down a drill string having a drill bit on the lower end thereof, and through ports provided in the drill bit to the well bottom and thence back to the surface through the annulus formed about the drill string. The drilling fluid serves to cool the drill bit, to transport drill cuttings to the surface, and to stabilize the wellbore.
A problem often encountered in the drilling of a well is the loss of unacceptably large amounts of drilling fluid into subterranean formations penetrated by the well. This problem is often referred to generally as “lost circulation”, and the formations into which the drilling fluid is lost are often referred to as “lost circulation zones” or “thief zones”. Various causes may be responsible for the lost circulation encountered in the drilling of a well. For example, a formation penetrated by the well may exhibit unusually high permeability or may contain fractures or crevices therein; In addition, a formation may simply not be sufficiently competent to support the pressure applied by the drilling fluid and may break down under this pressure and allow the drilling fluid to flow thereinto. An additional problem associated with drilling through a high permeability formation using a drill bit attached to the lower end of a drill string is that occasionally the drill string becomes stuck and cannot be raised, lowered or rotated. There are numerous causes for this problem, one of the most common being differential sticking. Differential sticking usually occurs when drilling through a permeable formation where the borehole pressure is greater than the formation pressure and when the drill pipe remains at rest against the wall of the borehole for enough time to allow a filter cake comprised of drilling fluid solids to build up around the pipe. The pressure exerted by the drilling fluid then holds the pipe against the filter cake. A reduction in fluid loss from a drilling fluid would reduce the thickness of the filter cake, thus reducing the incidence of differential sticking. Damage (productivity loss) is caused by the invasion of fluids into producing formations associated with the loss of filtrate from drilling fluids and from other types of wellbore fluids such as completion fluids, workover fluids and packer fluids. It would therefore be desirable to reduce the fluid loss from a wellbore fluid into a subterranean formation, in particular, the fluid loss from a drilling fluid into a subterranean formation.
Wellbore fluid compositions, in particular drilling fluid compositions are known to be flowable systems that are generally thickened to a limited extent. Wellbore fluids can be assigned to one of the three following classes: wellbore fluids based on oil which as a rule are used in the form of so-called invert emulsion fluids, and represent preparations of the water-in-oil emulsion type in which the aqueous phase is distributed as a heterogeneous fine dispersion in the continuous oil phase; purely aqueous based wellbore fluids; and aqueous based wellbore fluids of the oil-in-water emulsion type in which the oil phase is distributed as a heterogeneous fine dispersion in a continuous aqueous phase. A disadvantage of wellbore fluids based on oil is that the oil tends to coat the drill cuttings which creates an environmental problem, especially in offshore drilling operations, when the drill cuttings are discarded. To avoid serious pollution, the oil must be cleaned from the cuttings before they can be dumped. It is difficult and expensive to reduce the oil contamination to currently permitted levels, but even these small amounts of residual oil are ecologically undesirable, and there is pressure to replace oil based wellbore fluids with aqueous based ones. However, the technical properties and attributes of aqueous based fluids are significantly different from those of oil based fluids, and the challenges of developing aqueous based fluids are considerable. The present invention relates to improved aqueous based systems, that is, systems having a continuous aqueous phase, i.e. either purely aqueous based fluids, or oil-in-water emulsions.
Wellbore fluids often contain polymers performing various functions. Polymers are commonly added in order to modify the various properties of the fluid, for example, to increase the viscosity of the fluid. For example, WO 2005/105949 discloses a drilling fluid composition comprising a non-aqueous base fluid, a blend of one or more copolymers, and polyethylene. The combined use of the polyethylene and a copolymer leads to improved flow properties and gel strengths. The technology is only applicable to non-aqueous based fluids.